Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having high-energy density and high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
Generally, the lithium secondary battery uses a transition metal oxide such as LiCoO2 as a cathode active material and a carbonaceous material as an anode active material, and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode and addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into the carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Such a lithium secondary battery must be basically stable in an operating voltage range of the battery and must have an ability to transfer ions at a sufficiently rapid rate.
However, repeated charge/discharge of such a lithium secondary battery results in dissolution of the cathode active material as metal components at the cathode side and precipitation of such metal components at the anode side, which consequently causes the problem of electrolyte decomposition on the surface of anode. The dissolution and precipitation of the metal components and the electrolyte decomposition become severe during high-temperature storage of the battery, thereby resulting in decreases in residual capacity and recovery capacity of the battery.
In order to solve the problems associated with decreased residual capacity and recovery capacity of the battery, a variety of techniques have been introduced for preventing the dissolution of the metal components of the cathode active material in the lithium secondary battery. For example, there has been proposed a technique of decreasing the dissolution of metal elements over time into the electrolyte by reducing a specific surface area of the active material (Y. Xia, et al. J. of Power Source 24, 24-28 (1998)). However, this method involves preparation of active material particles into a large size, which in turn requires long-term heat treatment, and also disadvantageously suffers from deterioration of high-rate discharge characteristics and low-temperature characteristics of the electrode due to a decreased reaction area and an increased diffusion distance of lithium ions.
In addition to such a technique, a surface treatment method is widely used. For example, in order to improve high-temperature storage characteristics, a surface treatment technique using heterogeneous elements (Li2CO3, Na2CO3, K2CO3 and the like). However, such a surface treatment method also suffers from shortcomings in that severely decreases battery capacity and adds multiple manufacturing processes.
Therefore, there is a strong need in the art for the development of a technique capable of fundamentally solving the above-mentioned problems.